It is well known in the art of refrigeration control systems to use a pressure control switch which responds to pressure conditions in the suction line between the evaporator and compressor. The purpose of this device is to act as a safety control to prevent the suction pressure from falling to a point where the compression ratio will be too great for the compressor design. It also prevents the suction pressure from falling to a point where other damage can occur from low temperature. This type of controller normally has an adjustable range or differential and serves to turn off the compressor at the lower end of the range (cut-out) when pressure (and therefore temperature) conditions have been satisfied. At the upper end of the range (cut-in), the compressor is turned on again. The average pressure (and temperature) of the suction line is therefore maintained at the average of cut-in and cut-out levels of the pressure control switch.
It should be noted that a temperature control switch such as a thermostat placed in the refrigeration compartment containing the evaporator will perform a similar function to that of the pressure control switch. The temperature control switch will normally have a temperature sensing means, and have cut-in and cut-out settings similar to that of the pressure control switch.
Since many refrigeration systems are required to maintain operation over a wide range of temperature conditions, the pressure control switch is subject to certain seasonal problems. During winter periods, compartment cooling may be required even though the outdoor temperature is quite low. Under such conditions, the condenser may condense refrigerant so effectively that the compressor is unable to maintain a satisfactory hot gas pressure for delivering sufficient refrigerant across the restriction between the high and low sides of the system. As a result, the evaporator may be semi-starved of refrigerant, especially at start-up. At start-up, the compressor may draw more refrigerant from the evaporator than can be replaced by the condenser, thus causing the suction line pressure to suddenly drop, thereby actuating the low pressure cut-out. The result is a short on-off-on cycling of the compressor which can place excessive strain and loading on the compressor motor.
In the warm summer months where energy conservation is extremely important, high ambient temperatures generally cause the compressor to run for very long periods followed by very brief off periods.
In most cases, it is desirable not to have to adjust the pressure (or temperature) limits and range of the pressure control switch seasonally. For example, ambient temperature conditions can at times change very rapidly, necessitating a high degree of alertness and maintenance care, if the refrigeration control system is not adaptible to these changing conditions.
To surmount these problems posed by changing temperature conditions in the vicinity of the refrigeration system, various modifications and improvements have been introduced to the art.
To avoid the short-cycling problem, the pressure control switch differential between cut-in and cut-out has often been increased. This, however, causes much longer on-off cycles than desired, especially under high ambient conditions where energy conservation is of prime importance.
In order to arrive at a satisfactory compromise to the problem of short-cycling and excessive compressor run-time, various refrigeration control devices have included a fixed compressor off-time delay implemented by either electro-mechanical or solid-state devices. This off-time delay serves to allow sufficient pressure (and temperature) to build up in the suction line between evaporator and compressor, and thus prevents short-cycling. The disadvantage to the fixed off-time delay control is that no upper pressure limit is sensed prior to turning on the compressor. The average temperature of the refrigeration unit is thus allowed to vary excessively at the expense of providing short-cycle prevention.
U.S. Pat. No. 4,292,813 attempted to surmount this problem posed by the fixed time delay by adjusting the off-time delay in inverse manner to the latest recorded compressor run-time. That is, if the last compressor run-time was lower than some minimum desired time, the off-time was increased. Similarly, if instead, the run-time was recorded to be greater than some maximum desired time, the off-time delay was increased. The object of changing the off-time delay in this manner was to force the next compressor run-time to track the off-time, thus maintaining a fairly constant duty cycle. This adaptation does tend to compensate for the extreme ambient temperature operating conditions of the refrigeration system, but like other off-time delay methods, it does not start the compressor motor at a fixed upper pressure (and temperature). The average refrigerator temperature, therefore, is not controlled as closely as may be necessary.
The prior art searched did not disclose any patents or publications that were directly related to a refrigeration system energy controller of the type disclosed herein. However, the following U.S. patents were considered in the investigation and evaluation of the prior art relative to the existing apparatus used with the invention:
______________________________________ U.S. PAT. NO. INVENTOR ISSUED ______________________________________ 4,292,813 Paddock 6 October 1981 4,142,375 Abe, et al 6 March 1979 4,094,166 Jerles 13 June 1978 3,864,611 Chang 4 February 1975 3,636,369 Harter 18 January 1972 ______________________________________
The Paddock patent describes an adaptable off-time delay associated with an ambient temperature thermistor sensor used to sense where the compressor should be turned on. Abe, et al, describe a fixed off-time delay associated with an auxiliary fan for cooling of evaporator coils during compressor off-time. The Jerles patent describes fixed on and off-time delays for compressor operation. The Chang patent describes a solid-state control circuit providing a fixed off-time delay. The Harter patent describes a provision for a 2-minute delay during the initial portion of the run cycle to provide enough time for lubrication, in addition to providing a fixed 5-minute off-time delay.